Are there any bigger companies that do a lot of pick-n-pack in your area? I've found that employees from these types of places tend to be great hires...if you call around to temp agencies and see who has worked for a particular company (in my area it was Honda), you might have some luck. Lots of these places go with temp agencies when they have an influx of work and need staff immediately - that way, your hires are already familiar with the hustle and bustle of high-volume pick-n-pack.
Another option is incorporate metrics into your pick-n-pack process (say THAT three times fast...) I've seen companies that have barcode/binning systems and pick carts with laptops. The laptop tells them what row to go into, what bin the item is in (so has to be integrated with the inventory and bin-assignment system). If you have a huge warehouse/inventory, then it'll be a good idea to make sure the scanning software sends them on the most efficient route. The scanning will give a different sound if the correct/incorrect item is scanned, and once it is correctly scanned and picked, it will tell the picker to go to the next item and where that item is located. The employees had goals they couldn't fall below - eg, X amount of items scanned per hour, get spiffs/incentives if they go above Y items per hour, etc.
Not sure if your pickers also do the packing, but if you're doing enough volume, you'd probably want to split it up. When the packers scan the package/barcodes to close-out/weigh the package, they will have goals to hit as well.